1. Field of the Invention
This invention relates to low-voltage electromechanical devices including a tiltable microplatform, methods of tilting same, arrays of such devices, and methods of setting dimple-to-substrate spacing.
2. Background Art
In the wake of rapid advancements in optical MEMS technology spurred by the telecommunications industry, micromechanical actuation mechanisms capable of generating plate tilt angles on the order of 10° are now commonplace. Among the various actuation strategies, vibromotor and bimorph methods stand out as some of the most voltage efficient, requiring 20V AC and 1-2 V (but with considerable power consumption), respectively, to attain tilt angles greater than 10° in large plates, often for optical scanning applications. However, actuation methods capable of low-power, low-voltage tilt actuations of plates that allow light to pass through a 3D array of elements are fewer in number and most require excessive power or voltage levels.
Torsional micromechanical devices have large application prospect especially in micro-optics areas. Traditionally, the tilting of a device is achieved via applying electrostatic force between a plate suspended over the substrate by two torsional beams and electrodes under the plate. Unfortunately, devices based on this mechanism suffer from the limitation of tilting angles because of the hindrance of the substrate and large driven voltages to overcome the mechanical torque of the supporting beams.
U.S. Pat. No. 4,317,611 and the article by K. E. Peterson entitled “Silicon Torsional Scanning Mirror,” IBM J. RES. DEV., 24(5), 1980, pp. 631,637 both disclose silicon torsional micromachined mirrors wherein the mirror and torsion elements were patterned in a thin (134 microns) silicon wafer and retained the full thickness of the wafer. The structure was then bonded to a glass substrate, over a shallow well to allow room for the mirror motion. Actuation of the device was electrostatic. The mirror body was used as one electrode and the other electrodes were placed at the bottom of the well under the mirror. A narrow ridge in the well under the axis of rotation of the mirror was used to eliminate transverse motion of the structure. The manufacturing process for this device was relatively simple, requiring a single patterning step for the silicon and two patterning steps for the glass substrate. Its resonance frequency was about 15 kHz, and at resonance the angular displacement reached about 1°. The limitations of this device are related to the depth of the well. A 2 mm mirror touches the bottom of a 12.5 μm well at a displacement of 0.7° (1.4° total motion).
Nelson (U.S. Pat. No. 5,233,456), Baker et al. (U.S. Pat. No. 5,567,334), Hornbeck (U.S. Pat. No. 5,552,924), and Tregilgas (U.S. Pat. Nos. 5,583,688 and 5,600,383) have developed and patented a series of torsional mirror designs and improvements for use in deformable (or digital) mirror device (DMD) displays. These mirrors are fabricated by surface micromachining, consisting of a series of patterned layers supported by an undisturbed substrate. The DMD display uses an individual mirror at each pixel. The mirrors are therefore designed to be very small, to be operated in a bi-stable mode, and to maximize the packaging fraction on the surface of the display. To minimize the gaps between the reflecting surfaces of adjacent mirrors, the support structure and drive components are fabricated in underlying layers, requiring a multi-step deposition and patterning process.
As with the Peterson mirror, the Hornbeck mirror is designed to serve as one of the deflection electrodes, and the others are placed behind the mirror. Owing to the small size of the mirrors (about 20 μm×20 μm), high deflection angles are attainable with reasonably small gaps. These mirrors are designed for driving at low frequencies, and for significant dwell at a given angle (on or off), rather than for continuous motion, although the early development included mirrors designed for resonant operation (U.S. Pat. No. 5,233,456). A scanned display or imager requires, however, a large mirror.
U.S. Pat. No. 5,914,801 discloses a torsional structure which has a tiltable plate hung over a hole in the substrate, eliminating the tilting angle limitation. However, the voltage needed to actuate the device is still large, and the tilting angle depends on how close the acting point on the plate can be to the central axis, which may be limited by the fabrication process. In particular, the mirror body is formed from the silicon substrate, while the supports and actuators are fabricated above the mirror plane using surface micromachined polycrystalline silicon layers. (Also, V. J. Dhuler, “A Novel Two Axis Actuator for High Speed Large Angular Rotation,” Conference Record of “TRANSDUCERS '97”, 1997.) The mirror body is first defined using ion implantation of boron as an etch stop, and then by removal of the excess Si wafer from the back of the mirror. The supports and drive electrodes are offset from the top surface of the substrate by posts, which define the gap between the drive capacitor plates. Thus, the mirror is free to rotate unhindered by the bottom surface of a well, while the drive torque, being applied by actuators, is not limited by a requirement for a large capacitor gap. In the process, the mirror body thickness is limited by the boron implantation process, which has limited penetration depth; the disclosed mirror was 4 μm thick. The stiffness of the mirror is limited by both its size and thickness, so larger mirrors need to be thicker to avoid deformation of the mirror surface in use. For scanning applications, flexure in the mirror leads to uncertainty in the pixel size and location and distortion of the pixel shape. The implantation process also introduces stress into the mirror body, causing deformation of the reflective surface. The supports and actuators of the device are formed in a multi-step process and, as they are non-conducting, require the separate deposition and patterning of electrodes.
A 200 μm×250 μm mirror that has a frequency of 15 kHz is disclosed in M. H. Kiang, “Surface Micromachined Electrostatic Comb Driven Scanning Micromirrors for Barcode Applications,” 9TH ANNUAL WORKSHOP ON MICRO ELECTRO-MECHANICAL SYSTEMS, 1996, San Diego, Calif., pp. 192-197. This mirror is made of deposited and patterned surface layers, and before use it must be first rotated out of the plane of the substrate using a comb drive and locked into position using complicated hinges. This approach obviates the problem of forming a cavity behind the mirror. However, the use of surface micromachined layers means that the structural rigidity of the micro-mirror cannot be controlled (because the thicknesses of the layers are limited to a few microns). The mirror motion is obtained by electrostatic drive applied by an actuator linked to one edge of the mirror. The motion of the mirror is restricted by the actuation mechanism.
The article by R. Legtenberg et al., “Electrostatic Curved Electrode Actuators,” JMEMS, Vol. 6, No. 3, pp. 257-265, September 1997 is related to the present invention.
U.S. Pat. No. 6,249,073 discloses a flexural-mode micromechanical resonator including a non-intrusive support structure and one or more spacers in the form of dimples found on a resonator beam. The dimples determine a capacitive-transducer gap of the resonator.
U.S. Pat. No. 6,545,385 discloses a micromechanical apparatus for elevating and tilting a platform using a plurality of flexible members which, in turn, are controlled by one or more MEMS actuators.